In digital wireless communication using a multi-level modulation system, as a modulation multi-level number becomes large, an information transmission amount per unit frequency becomes large, although communication becomes weak against disturbance in a channel. When quality of a channel becomes lower than a certain level, the number of transmission errors is increased, and communication is cut off sooner or later. There has been a request for securing communication of a minimum level even in such a case. In view of the above, there has been considered a communication system using an adaptive modulation method. In this method, a channel is monitored, and when deterioration in quality of the channel is detected, a modulation multi-level number is lowered to secure communication of good quality at a bit rate of a minimum level, and when quality of the channel is excellent, the modulation multi-level number is increased to enable communication of large capacity.
For example, in a system described in Patent Document 1, the system carries out control of selecting a modulation system that can maintain communication quality of a certain level from modulation systems that can be used in accordance with a state of a channel at a certain time point. For example, incase there are three multi-level modulation systems, which are 64 QAM (quadrature amplitude modulation), 16 QAM, and QPSK (quadrature phase shift keying), and when quality of a channel begins to be deteriorated, switching is made from 64QAM to 16QAM first, and if deterioration progresses further, switching is made from 16 QAM to QPSK.    Patent Document 1: JP2005-012684A